The invention is based on an apparatus and a method for operating a rain sensor.
From German published, nonexamined Patent Application DE-OS 41 12 847, an apparatus for operating a rain sensor is already known having a transmitter which is triggered by a preceding control arrangement, a receiver which outputs a sensor signal to a signal processor, and an evaluator, which outputs a switching signal for turning a windshield wiper on as a function of the sensor signal.
A regulator is also provided that regulates the sensor signal, which corresponds to a clean window, to a predetermined resting level. To that end, the controller outputs a control signal to the control arrangement of the transmitter for the sake of slowly regulating the transmission power to a predetermined resting level. Alternatively, the controller outputs a control signal to the signal processor for slowly regulating the gain of the sensor signal to be amplified. With the regulator, production variations among individual components of the rain sensor as well as tolerances in rain sensor installation can be compensated for over a wide range.
A disadvantage is that the controller regulates the sensor signal corresponding to a clean, dry window to the resting level, preferably at the outset, and that over the further course of sensor operation the control signal of the controller is allowed to vary in comparison with the sensor signal only extremely slowly, so that changes in the sensor signal will not be cancelled out. That is, the regulator essentially performs a (one-time) calibration of the rain sensor.
This then means that the evaluation of the sensor signal takes place essentially in the working range that has been set. This has the disadvantage that at small sensor signals, any change in the sensor signal has poorer resolution than an equally major relative change in large sensor signals.
In accordance with the present invention, the control signal outputted by a controller as a function of the sensor signal is also delivered to an evaluation arrangement for treating switching events of the device.